Flame-retardant acrylic synthetic fibers having improved properties

ABSTRACT

Flame-retardant acrylic synthetic fibers containing a fluid mixture obtained by dissolving halogen- and/or phosphoruscontaining solid organic compounds in a liquid halogenated aliphatic phosphate.

United States Patent im Takeya et al. Nov. 5, 1974 FLAME-RETARDANT ACRYLIC [58] Field of Search. 260/45.7 P, 4595 G, 45.95 R, SYNTHETIC FIBERS HAVING IMPROVED 260/45.7 R, 28.5 A PROPERTIES [75] Inventors: Kenji Takeya; Hiroshi Suzuki; [56] References C'ted Tadashi Ichimaru, all of Okayama, UNITED STATES PATENTS Japan 3,149,089 9/1964 Hayes 260/45.7 P 3,645,964 2/1972 DiPietro [73] Assgnee' Japan Exla m 3,658,634 4 1972 Yanagi eta] zoo 45.95 o

Osaka, Japan [22] Filed: Sept. 1, 1972 Primary ExaminerMelvyn l. Marquis [2]] pp No: 285,664 Attorney, Agent, or F1rm-Wenderoth, Lind & Ponack 4 [57] ABSTRACT [3O] Forelgn Apphcamn Pnomy Data Flame-retardant acrylic synthetic fibers containing a Sept. 4, 1971 Japan 46-68218 fluid mixture Obtained by dissolving halogen and/or phosphorus-containing solid organic compounds in a [52] US. Cl. 260/45.95 G, 260/28.5 A, 2.60/45] R, liquid halogenated aliphatic phosphate 260/45.7 P, 260/45.95 R, 264/182 Int. Cl C08g 51/58 9 Claims, No Drawings I FLAME-RETARDANT ACRYLIC SYNTHETIC FIBERS HAVING IMPROVED PROPERTIES The present invention relates to flame-retardant acrylic synthetic fibers with improved chemical and physical properties, and also to a method for producing such acrylic synthetic fibers. More particularly, in producing flame retardant acrylic synthetic fibers containing a liquid halogenated aliphatic phosphate, the pres ent invention relates to a method for improving the various properties of the fibers such as touch, luster, antifibrillation property, heat-settability, resilience, etc. and for augmenting the flame retardant property of the fibers to a further extent as well as for preventing the fluctuation in dyeability, characterized in that a fluid mixture having a viscosity of at least 6,000 centipoises at 25C obtained by dissolving in the halogenated aliphatic phosphate a solid organic compound which is soluble in the halogenated aliphatic phosphate and contains halogen and/or phosphorus, is introduced into the fibers.

It is well known that acrylic synthetic fibers can be rendered flame retardant by introducing a halogenated aliphatic phosphate into the fibers. When a proper amount of a halogenated aliphatic phosphate is introduced into the fibers, the resultant fibers certainly show not only desirable flame retardant property but also good touch, good luster and other good properties. However, if the halogenated aliphatic phosphate to be introduced into the fibers is increased to such an amount as to make the fibers more flame-retardant, the flame retardant property is further improved, but other fiber qualities and properties will deteriorate with the increase of the amount of the flame retardant additive. Therefore, the various disadvantages resulting from the halogenated aliphatic phosphate introduced into the fibers, such as the fibrillation caused in the steps of forming the fibers containing the halogenated aliphatic phosphate, lack in resilience of the products in use, change of touch with the passage of time, etc. were inevitable when a large amount of the halogenated aliphatic phosphate was introduced.

The above-mentioned phenomenon can be more concretely explained from the following fact: Thus, by introducing the above-mentioned halogenated aliphatic phosphate into the fibers, it is possible to obtain acrylic synthetic fibers for use in artificial hair 'having excellent touch and luster like human hair and a flame retardant property. However, when the fibers are sewed by machine in the step of preparing hair prod ucts or when the products in use are combed or brushed, there will be formed undesirable fibrillated, branched or worn-out hair, or when the products are washed the curls and waves of the artificial hair tend to vanish. Therefore, in such products there is a problem of deteriorating the excellent touch and luster of the fibers like human hair with the passage of time during wearing. It is also a problem that the products are not durable to a long time wear because of the lack in resilience of the curls and waves or the loss of resilience -with the passage of time. t

In order to avoid the disadvantages of deterioration of the fiber qualities and properties resulting from introducing a large amount of the above mentioned flame retardant additive into the fibers, attempts have been made to reduce the amount of the halogenated aliphatic phosphate to be added to the fibers by using, to-

gether with the halogenated aliphatic phosphate, a solid flame retardant additive such as antimony oxide which is entirely incompatible with the halogenated aliphatic phosphate. However, a flame retardant additive such as antimony oxide, which'has no interaction with acrylonitrile polymer in the fibers, has a disadvantage of being more liable to cause the problem of fibrillation than in the case of adding the halogenated aliphatic phosphate only.

We have now found that the viscosity of the flame retardant additive to be introduced into the fibers remarkably influences the properties of the products, and further found that when a fluid mixture prepared as to have a viscosity of more than 6,000 centipoises by dissolving a particular organic compound containing halogen and/or phosphorus into the halogenated aliphatic phosphate is introduced into acrylic synthetic fibers, the fiber properties are remarkably improved, and the flame retardant property is synergistically improved.

The main object of the present invention is to improve the properties of flame retardant acrylic synthetic fibers.

An object of the present invention is to obtain acrylic synthetic fibers which are improved not only in the properties including touch, luster, anti-fibrillation, heat-settability, resilience, etc., but also effectively prevented from the fluctuation in dyeability, and moreover rendered highly flame retardant.

Another object of the present invention is to improve the various properties of acrylic synthetic fibers and to augment the flame retardant property to a further extent as well as to prevent the fluctuation of dyeability by incorporating a fluid mixture having a viscosity of at wherein each of R R and R stands for a halogenated aliphatic hydrocarbon radical having no more than 8 carbon atoms, the above-mentioned objects of the present invention can be attained by incorporating into the fibers a fluid mixture "having a viscosity of at least 6,000 centipoises at 25C-obtained by dissolving in the halogenated aliphatic phosphate a solid organic compound containing halogen and/or phosphorus which is soluble in the halogenated aliphatic phosphate.

The thus-obtained flame retardant acrylic synthetic fibers are excellent not only in the flame retardant property but also in other properties including touch, luster, anti-fibrillation, heat-settability, resilience,-etc. Therefore, for example, artificial hair products having I excellent toucn and luster like human hair and excelcausing fibrillated, branched or worn out hair upon combing or brushing, and of tending to lose the curls and waves by washing, i.e., free from the changes in the excellent human hair-like touch and luster with the passage of time during wearing.

It is to be noted that the acrylic synthetic fibers according to the present invention shows a much higher flame retardant property than that of the conventional acrylic synthetic fibers containing only a halogenated aliphatic phosphate when the amount of the flame retardant additive incorporated into the fibers is the same.

It is also to be noted that the acrylic synthetic fibers obtained according to the present invention do not show any fluctuations in dyeability as contrasted to the conventional acrylic synthetic fibers containing only a halogenated aliphatic phosphate.

Additionally, the flame retardant acrylic synthetic fibers obtained according to the present invention, when used for preparing artificial hair, can give more excellent human hair-like luster, by using a compound having a refractive index at 25C ranging from 1.44 to 1.60 as the above-mentioned halogenated aliphatic phosphate. 1

The halogenated aliphatic phosphate to be used in this invention and represented by the above indicated general formula include liquid compounds such as tris( ,B-chloroethyl)phosphate, tris(B-bromoethyDphosphate, tris( 3-chloropropyl)phosphate, tris( 3- bromopropyl) phosphate, tris(2- chloroprOpyDphosphate, tris( 2- bromopropyl)phosphate, tris(2,3- dichloropropyl)phosphate, tris(2,3-

tris( l-bromo-3- tris( 2,3- tris( 2,3-

dibromopropyl)phosphate, chloroisopropyl )phosphate, dichlorobutyl )phosphate,

dibromobutyl)phosphate, bis(2,3-d ichloropropyl) 2,3-

wherein each of R and R is alkyl, cycloalkyl, aralkyl or aryl group which is unsubstituted or substituted with halogen atom(s) and which has 2 to 20 carbon atoms, such as bis(benzyl)ether of tetrabromobisphenol-A, bis(chlorobromopropyl)ether of tetrabromobisphenol- A, etc.; precondensates of tetrabromobisphenol-A with polyhalogenated compounds or diepoxides, etc.; halogenated diphenyl ethers such as chlorinated'diphenyl ether, brominated diphenyl ether, etc.; halogenated phenols or its halogenated alkyl (preferably not more than 10 carbon atoms) ethers, etc. Besides these, any other solid organic compounds containing halogen and/or phosphorus and soluble in the halogenated aliphatic phosphate can be used.

Such a halogenated aliphatic phosphate and an organic compound containing halogen and/or phosphorus may be mixed in various combinations and dissolved to form a homogeneous fluid mixture having a viscosity of at least 6,000 centipoises. Any mixing ratio (by weight) of such a halogenated aliphatic phosphate and such organic compounds containing halogen and- /or phosphorus can be used, if this ratio gives a fluid mixture satisfying the foregoing viscosity. However, in general, it is desirable to use a ratio within the range of from 9.5205 to 0519.5, preferably 9/l-3/7.

By the term fluid mixture as used herein is meant a homogeneous composition which is fluid at room temperature'or upon the operation of introducing the mixture into the fibers. Also, it is an essential requirement that the viscosity of the fluid mixture should be at least 6,000 centipoises at 25C. When a fluid mixture of less than that viscosity is used, it is difficult to fully attain the objects of the present invention. The upper limit of the viscosity of the fluid mixture according to the present invention may vary depending on the particular combination of the halogenated aliphatic phosphate and the organic compound containing halogen and/or phosphorus to be used or on the degree of the desired modifying effect of the fiber. Even a composition which is solid at room temperature but is fluid upon introducing it into the fibers can be used. However a viscosity of less than 300,000 centipoises is generally desirable for operation.

By introducingsuch a fluid mixture into the fibers in an amount of not less than 3 percent on the weight of the fibers, it is possible to obtain acrylic synthetic fibers having a high flame retardant property and improved properties in various other respects. In general, the upper limit of the amount of the fluid mixture to be introduced into the fibers is desirable to be not to be more than 40 percent in consideration of the other fiber properties.

The most advantageous procedure to introduce such a fluid mixture of halogenated aliphatic phosphate and organic compounds containing halogen and/or phosphorus into the acrylic synthetic fibers are to incorporate a fluid mixture having a viscosity within the abovementioned specified range into the spinning solution for producing the fibers. Then the spinning solution containing said fluid mixture is formed into filaments by a generally well known wet spinning or dry spinning method. The spun filaments may further be treated in a known manner. Thus, the filaments may be waterwashed, stretched, dried and heat-treated. Another procedure for causing the above-mentioned fluid mixture to be contained in the fibers is to treat the gel filaments (obtained by dryor wet-spinning an acrylic polymer solution through spinneret orifices under general spinning conditions) in a treating bath containing a fluid mixture of a viscosity within the abovementioned specified range, and then to carry out the usual post treatment steps. Thus there can be various procedures to cause the above-mentioned halogenated aliphatic phosphate and organic compound containing halogen and/or phosphorus to be contained in the fibers according to the present invention. By any of these other U.S. Patents cited therein.

The representative compounds which may be copolymerized with acrylonitrile to produce acrylonitrile polymers useful for the practice of the present invention are those containing one c11 =c (radical.

Such compounds include, for example, vinyl esters of saturated aliphatic carboxylic acids such as vinyl acetate, vinyl propionate, vinyl butyrate, etc.; vinyl halides and vinylidene halides such as vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene chloride, vinylidene bromide, vinylidene fluoride; allyl type alcohols such as allyl alcohol, methallyl alcohol, ethallyl alcohol, etc.; allyl, methallyl and other unsaturated alcohol ester of monobasic acids such as allyl or methallyl acetate, laurate, cyanide, etc.; acrylic acid, alkacrylic acids (such as methacrylic acid, ethacrylic acid, etc.), and esters and amides of such acids (such as methyl, ethyl, propyl, butyl acrylates and methacrylates; acrylamide, methacrylamide, N-methyl, -ethyl, -propyl, -buty1 acrylamides and methacrylamides); methacrylonitrile, ethacrylonitrile, and other hydrocarbon substituted acrylonitriles; unsaturated sulfonic acids having one CH2=C radical and their salts such as allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid and their sodium salts and potassium salts; unsaturated aliphatic hydrocarbon having one CH =C (radical such as isobutylene; many other vinyl, acrylic and other compounds having one CH =C (radical which polymerize with acrylonitrile to produce thermoplastic copolymers. Alkyl esters of a,B-unsaturated polycarboxylic acids such as dimethyl, -ethyl, -propyl, -butyl esters of maleic acid, fumaric acid, citraconic acid also copolymerize with acrylonitrile to form copolymers.

Usually, the. molecular weight (average molecular weight) of the acrylonitrile homopolymer or copolymer for producing polyacrylonitrile shaped products is within the range of from 25,000 or 30,000 to 200,000 or 300,000, or higher, and the particularly advantageous range is from 50,000 to 100,000. These molecular weights are calculated from the viscosity of the polymer in dimethylformamide calculated by the Staudingers equation (refer to the specification of U.S. Pat. No. 2,404,713 dated June 23, 1946.).

Preferably, the acrylic polymer to. be used in this invention is an acrylonitrile homopolymer or copolymer containing 30 percent or less of at least one of the above mentioned monoethylenically unsaturated monomer and at least percent acrylonitrile, but polymers containing a less amount of acrylonitrile may also be used for the practice of the present invention.

The representative solvents which are useful to dissolve the acrylonitrile polymer to prepare thespinning solution include organic solvents such as dimethylformamide, dimethylacetamide, ethylene carbonate, and dimethyl sulfoxide, and inorganic solvents such as concentrated aqueous solution of inorganic salts, for example sodium thiocyanate, zinc chloride, etc.

The spinning methods used for obtaining the fibers of the present invention may be the generally known wet or dry spinning processes described in Japanese Patent Publication Nos. 3645/50, 4821/53, 9516/57, 878/63 and 2589/61, and U.S. Pat. Nos. 2,404,725 to 2,404,728.

The following examples are for better explanation of the present invention and are not intended to limit the scope of the invention. All the percentages and parts in the examples are by weight unless otherwise indicated.

EXAMPLE 1 A fluid mixture having a viscosity of 230,000'centipoises at 25C consisting of 70 parts tris(l-bromo-3- chloroisopropyl)phosphate having a viscosity of about 3,000 centipoises at 25C and 30 parts chlorinated paraffin containing 70 percent chlorine was prepared. The fluid mixture was then added to a spinning solution composed of 11 parts of a copolymer consisting of 88 percent acrylonitrile and 12 percent vinyl acetate and 89 parts of a 44 percent aqueous solution of sodium thiocyanate, the amount of the fluid mixture being 40 percent on the basis of the weight of the polymer. Thereafter, the fluid mixture was thoroughly dispersed in the spinning solution by a high shear mixer (Homomixer SL Type, manufactured by Tokushu Kika Co., Japan). Thus thus-obtained spinning solution was then extruded through a spinneret having eight orifices of each 0.2 mm in diameter into a 12 percent aqueous solution of sodium thiocyanate at -2C. The formed filaments were stretched ten times the original length in hot water, washed with water and then steam-relaxed at l 10C to obtain acrylic synthetic fibers of 50 denier per filament.

The thus-obtained acrylic synthetic fibers were evaluated for flammability to show extremely high self-extinguishability. Further, no fibrillation was observed upon dyeing, cutting and sewing the fibers to prepare artificial hair products.

For comparison, when tris( l-bromo-3- chloroisopropyl)phosphate was singly introduced into the fibers instead of the above-mentioned fluid mixture, the flame-resistance of the thus-obtained fibers was observed to be much inferior to that of the fibers according to the present invention. Furthermore, in the sewing step for producing artificial hair products, the

fibers in contact with the sewing machine turned white, which showed the occurrence of fibrillation.

EXAMPLE 2 in table 1 are shown several properties of various spinning solution thoroughly byahigh shear mixer. The acrylic synthetic fiber obtain d by the th d as thus-obtained spinning solution was then extruded into shown in Example 1 in which va i a o nts f th a 12 percent aqueous solution of sodium thiocyanate at conventional flame retardant additive tris( l-bromo-3- 2C through a spinneret having 50 orifices of each 0.2 chloroisopropyl) phosphate singly as well as a h g mm in diameter. The formed filaments were stretched neous fluid mixture having a viscosity of 24,400 centi- 10 times the Oflglha] length in hot Water, Washed With poises at 25C consisting of 70 percent said phosphate ater and then Steam-relaxed at l 15C to Obtain and 30 percent bis(2,3-dibr0m pr py1)eth f t t acrylic synthetic fibers of three denier per filament. bromobisphenol A were introduced i t th fib After combing the thus-obtained fibers 500 times, con- As apparent from the results in Table 1, it is observed Siderable brillation WaS observed. I that, by the method of the present invention, the acrylic 15 On the other h a fluid m xture having a v1sc0s1ty synthetic fibers containing the conventional haloge- Of 6,800 Cent1po1ses at 25C cons1st1ng of i0 parts of nated aliphatic phosphate can be remarkably im d the above-mentioned flame retardant additive and 30 in the fiber properties including the flame retardant P i f P R y ether was P p This property and dyeability. flu1d m1xture was used instead of the above-mentioned In the above tabl the flammability w d t i d flame retardant additive, and acrylic synthetic fibers of by filling 4 g. of the fibers into a metal wire basket (6.5 three dehlef P filament were ohialned 1n the Same cm. in diameter and 7.5 cm. in height), and contacting Y- F h the thus'obtalhed fibers, I10 3 burner fl to h bottom f h b k f 7 stantlal fibrlllation was observed even after 500 times onds. The burning time and burnt off weight were mea- Comblhgo sured. The fibrillation was determined as follows. Thus T Same Procedufg 'f p except that fhllfi a bundle of 800 monofilaments 40 cm. in length) was e er-e havmg a viscosity at 25 C of 26,000 centisubjected to 200 times brushing and the number of fl- POlSeS f Cohslsted of 70 Pement 0f tfl$( h brillated monofilaments was counted. The dyeability chlorolsopropyl)PhQSPhate and Percent Of (3- was determined as follows. Thus the fibers were dyed bromoethyhether of Ph W used under the f ll i conditions; as the fluid to be added to the splnning solut1on. The resultant fibers were excellent in flame-retardancy, Cationic dye: C.1. Basic Orange 21 (CI. 48035) hand (touch), lustre and even dyeability as compared 2% with the fibers obtained by the use of each of the above tasties two oomoooooo singly- L' t' 1/100 Ti r igr a ttife: lncreasedcfrom C. to 100C. (at a EXAMPLE 4 r 1 d ib ti for 12m? mamtame at A fluid m1xture havlng a v1sc0s1ty at 25C of 46,000

centipoises and consisting of percent of bis(2,3- After dyeing, the presence of unevenness in dyeing was dichloropropyl)2,3-dibromopropyl phosphate and 30 b d p 40 percent of bis (B-bromoethyUether of tetrabromobis- Table 1 Content Flammability Fibril Stickiness Unevenness of lation in dyed additive Burning Weight colors time in burnt seconds off Introduction 28% 17 33% 13 Observed Observed r111 halog enated 26 52 77 Observed Observed aliphatic slightly slightly phosphate 23 59 81 Observed Observed only slightly slightly Introduction 28 3 29 none none none of the same after being 26 3 23 none none formed into the fluid 23 2 24 none none mixture of the present invention EXAMPLE 3 phenol-A was added to-an acrylic spinning solution consisting of 11 parts of an acrylic copolymer (92 per- 9 P Py )p p t which 18 cent acrylonitrile and 8 percent vinyl acetate) and 89 a halogenated ahphahc Phosphate flamfi retardant 65 parts of a 44 percent aqueous solution of sodium thioditive, was added to an acrylic spinning solution composed of l 1 parts of a copolymer consisting of percent acrylonitrile and 10 percent methylacrylate and cyanate, the amount of the fluid mixture being 10 percent based on the copolymer in the spinning solution. Then the fluid mixture was fully dispersed in the spinning solution by the use ofa high shear mixer. The spinning solution was extruded through a spinneret (50 holes, each 0.09 mm in diameter) into a 12 percent aqueous solution of sodium thiocyanate at 2C to form filaments. The filaments were stretched times the length, washed with water and steam-relaxed at 120C to obtain acrylic fibers (referred to as A) having 3 denier per monofilament.

For comparison, the same procedure was repeated except that bis(2,3-dichloropropyl)2,3-dibromopropyl phosphate alone or bis(B-bromoethyUether of tetrabromobisphenol-A alone was used as the additive for the spinning solution to obtain fibers B or fibers C respectively.

In respect of these fibers A, B and C flammability was evaluated in the same manner as in Example 2. It was observed that the fibers A are far more excellent in flame-retardancy than the fibers B and C. Further the fibers B had a somewhat sticky hand and the fibers C were extremely delustered and tended to fibrillation, but the fibers A of this invention were not sticky, excellent in hand and lustre and had no tendency to fibrillation.

The same procedure was repeated except that a fluid mixture having a viscosity at C of 9,200 centipoises and consisting of 70 percent of tris(2,3- dibromopropyl) phosphate and 0f percent bis(B- bromoethyl)ether of tetrabromobisphenol-A was used as the fluid to be added to the spinning solution. The resultant fibers were excellent in flame-retardancy, hand, lustre, anti-fibrillation, etc. as compared with the fibers obtained by the use of each of the above two compounds singly.

What we claim is:

l. A flame-retardant monocomponent acrylic synthetic fiber comprising an acrylonitrile polymer selected from the group consisting of homopolymers of acrylonitrile and copolymers containing at least 70 percent acrylonitrile and up to 30 percent of at least one monoethylenically unsaturated monomer copolymerized therewith, and having incorporated in such fibers 3-40 percent by weight of a fluid mixture having a viscosity of at least 6,000 centipoises at 25C obtained by dissolving (1) an organic compound selected from the group consisting of halogenated paraffins, halogenated aralkyl phosphates, halogenated aryl phosphates, halogenated diphenyl ethers, halogenated phenols, halogenated alkyl ethers of the halogenated phenols, tetrabromobisphenol-A and etherified derivatives of tetrabromobisphenol-A represented by the formula Br CH, Br J R 0 (I! ---OR Br CH Br wherein each of R and R respectively, represents alkyl, cycloalkyl, aralkyl or aryl which are unsubstituted or substituted by halogen, said R, and R groups respectively having 2-20 carbon atoms, in (2) a liquid halogenated aliphatic phosphate represented by the formula R0 P O 4. The acrylic synthetic fiber of claim 3 wherein said mixing ratio is within the range of from 9:1 to 3:7 by weight.

5. The acrylic synthetic fiber of claim 1 wherein said halogenated aliphatic phosphate is a liquid having a refractive index of 1.44 to 1.60 at 25C.

6. The acrylic synthetic fiber of claim 1 wherein said halogenated aliphatic phosphate is selected from the group consisting of tris(B-chloroethyl)phosphate, tris(B-bromoethyl)phosphate, tris(3- chloropropyl )phosphate, tris( 3- bromopropyl)phosphate, tris(2,3-dichloropropyl) phosphate, tris(2,3-dibromopropyl)phosphate, tris( 1- bromo-3-chloroisopropyl)phosphate, tris(2,3- dichlorobutyl) phosphate, tris(2,3- dibromobutyl)phosphate, bis(2,3-dichloropropyl)2,3- dibromopropyl phosphate and bis(2,3-dibromopropyl)2,3-dichloropropyl phosphate.

7. The acrylic synthetic fiber of claim 1 wherein said fluid mixture consists essentially of tris( l-bromo-3- chloroisopropyl)phosphate and bis(2,3- dibromopropyl)ether of tetrabromobisphenol-A.

8. The acrylic synthetic fiber of claim 1 wherein said fluid mixture consists essentially of tris( l-bromo-3- chloroisopropyl )phosphate and bis( [3- bromoethyl)ether of tetrabromobisphenol-A.

9. The acrylic synthetic fiber of claim 1 wherein said fluid mixture consists essentially of tris(2,3- dibromopropyl) phosphate and bis(B- bromoethyl)ether of tetrabromobisphenol-A. 

1. A FLAME-RETARDANT MONOCOMPONENT ACRYLIC SYNTHETIC FIBER COMPRISING AN ACRYLONITRILE POLYMER SELECTED FROM THE GROUP CONSISSTING OF HOMOPOLYMRS OF ACRYLONITRILE AND COPOLYMERS CONTAINING AT LEAST 70 PERCENT ACRYLONITRILE AND UP TO 30 PERCENT OF AT LEAST ONE MONOETHYLENICALLY UNSATURATED MONOMER COPOLYMERIZED THEREWITH, AND HAVING INCORPORATED IN SUCH FIBERS 3-40 PERCENT BY WEIGHT OF A FLUID MIXTURE HAVING A VISCOSITY OF AT LEAST 6,000 CENTIPOISES AT 25*C OBTAINED BY DISSOLVING (1) AN ORGANIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF HALOGENATED PARAFFINS, HALOGENATED ARALKYL PHOSPHATES, HALOGENATED ARYL PHOSPHATES, HALOGENATED DIPENYL ETHERS, HALOGENATED PHENOLS, HALOGENATED ALKYL ETHERS OF THE HALOGENATED PHENOLS, TETRABROMOBISPHENOL-A AND ETHERIIFIED DERIVATIVES OF TETRABROMOBISPHENOL-A REPRESENTED BY THE FORMULA
 2. The acrylic synthetic fiber of claim 1 wherein said fluid mixture has a viscosity of up to 300,000 centipoises at 25*C.
 3. The acrylic synthetic fiber of claim 1 wherein the mixing ratio of the halogenated aliphatic phosphate and the organic compound in said fluid mixture is within the range of from 9.5: 0.5 to 0.5:9.5 by weight.
 4. The acrylic synthetic fiber of claim 3 wherein said mixing ratio is within the range of from 9:1 to 3:7 by weight.
 5. The acrylic synthetic fiber of claim 1 wherein said halogenated aliphatic phosphate is a liquid having a refractive index of 1.44 to 1.60 at 25*C.
 6. The acrylic synthetic fiber of claim 1 wherein said halogenated aliphatic phosphate is selected from the group consisting of tris( Beta -chloroethyl)phosphate, tris( Beta -bromoethyl)phosphate, tris(3-chloropropyl)phosphate, tris(3-bromopropyl)phosphate, tris(2,3-dichloropropyl) phosphate, tris(2,3-dibromopropyl)phosphate, tris(1-bromo-3-chloroisopropyl)phosphate, tris(2,3-dichlorobutyl) phosphate, tris(2,3-dibromobutyl)phosphate, bis(2,3-dichloropropyl)2,3-dibromopropyl phosphate and bis(2,3-dibromopropyl)2,3-dichloropropyl phosphate.
 7. The acrylic synthetic fiber of claim 1 wherein said fluid mixture consists essentially of tris(1-bromo-3-chloroisopropyl)phosphate and bis(2,3-dibromopropyl)ether of tetrabromobisphenol-A.
 8. The acrylic synthetic fiber of claim 1 wherein said fluid mixture consists essentially of tris(1-bromo-3-chloroisopropyl)phosphate and bis( Beta -bromoethyl)ether of tetrabromobisphenol-A.
 9. The acrylic synthetic fiber of claim 1 wherein said fluid mixture consists essentially of tris(2,3-dibromopropyl) phosphate and bis( Beta -bromoethyl)ether of tetrabromobisphenol-A. 